This invention relates to semiconductor electronic devices, and, more particularly, to an approach for making low-electrical-resistance contacts to such devices.
A semiconductor device requires external connections from the semiconductor elements to wires, leads, or other metallic conductors which connect the semiconductor elements to other parts of a circuit. The semiconductors and the metallic conductors have fundamentally different electronic structures and electronic conduction mechanisms. The result of the different conduction mechanisms is the presence of a contact potential and energy barrier when the semiconductor is placed into contact with the metal. The contact potential adds a contact resistance to the flow of electrons in the circuit, which may be substantial and adversely affect the operation of the semiconductor device and the circuit containing the device. It is therefore usually desirable to reduce the contact resistance between the semiconductor and the metal to as low a value as possible.
An ohmic contact between a semiconductor and a metal is one where the contact resistance is a independent of the direction of current flow and is negligibly small. While ohmic contacts are readily created in a number of instances, in others such contacts are made only with difficulty. It is often difficult to make a satisfactory ohmic or near-ohmic contact to a wide bandgap semiconductor, because metals typically do not have a sufficiently small work function that leads to a small energy barrier.
The II-VI semiconductor materials include those based on ZnSe, ZnTe, and alloys derived from these compounds, often with additional elements such as mercury, cadmium, or magnesium from Group II of the periodic table, manganese from Group VII, or sulfur, selenium, or tellurium from Group VI. Some of these semiconductor materials are of significant commercial interest for use in the preparation of short wavelength diode lasers, as an example. These II-VI semiconductors exhibit relatively wide bandgaps of more than 2 electron volts. It is therefore difficult to form ohmic contacts to p-type semiconductor materials of this family, and in some cases no satisfactory ohmic contact structures are known. In those instances, the full potential of the II-VI semiconductor devices cannot be reached.
There is a need for an approach for achieving ohmic contacts to II-VI semiconductors such as ZnSe, ZnTe, their alloys, and doped structures based upon these materials. The present invention fulfills this need, and further provides related advantages.